1. Field of the Invention
This invention relates to piston or wrist pins for joining a pistonhead to a connecting rod of a piston and more particularly to piston pins having high performance characteristics to allow use with internal combustion engines. Further, this invention relates to a method of making this pin in an energy efficient manner.
2. Description of the Prior Art
Piston pins are in a well defined art and have been available for many years in various sizes and having various physical properties.
A piston pin is commonly used to pivotally join a pistonhead to a connecting rod of the piston for use in a positive displacement pump, an internal combustion engine or other such mechanical linkage converting linear movement to rotary movement or the reverse thereof.
U.S. Pat. No. 2,730,472 discloses a piston pin formed so that the pin has a hardened outer surface which is produced by heating the pin by induction coils. The interior of the pin remains sufficiently soft to allow a core or center opening to be subsequently formed therein. U.S. Pat. No. 2,604,419 discloses a hollow article formed by a similar method.
Another hollow piston pin with superior performance characteristics is made by carburizing the pins whereby the carbon content of the exterior surfaces is increased by placing the pin in a high carbon atmosphere to produce a hard exterior surface. The percentage carbon content of the surface of the pin may be increased to proximate 1.0% after a minimum 2-12 hour exposure.
Pins subjected to carburizing have several undesirable characteristics, however. Firstly, carburizing requires a high fossil fuel input. Secondly, the higher carbon content outer surface produces a pin having a reduced fatigue life.
As discussed in U.S. Pat. No. 3,216,869, improved fatigue life may be obtained by use of favorable residual stresses within a part. These residual stresses are achieved by producing a part having an outer zone which has a lower martensite forming temperature than an inner zone of the part. Where residual stresses are undesirable, the part may be "inverse hardened" as is discussed in U.S. Pat. No. 3,536,540.